Radiographic images such as X-ray images are employed in many fields, for example, for use in diagnosis of an illness. There have been employed, as a method for obtaining X-ray images, so-called radiation photography systems in which X-rays which have passed through an object are irradiated onto a phosphor layer (fluorescent screen) to form a visible light, which is irradiated onto a silver halide photographic material (hereinafter, also denoted simply as a photographic material) similarly to conventional photography, followed by being subjected to photographic processing to obtain a visible silver image.
Recently, there has been disclosed a new method for obtaining an image directly from a phosphor layer in place of an image forming method by use of silver halide photographic materials. In such a method, radiation having passed through an object is absorbed by a phosphor and then, the phosphor is excited by light or heat energy, whereby a radiation energy accumulated in the phosphor through the absorption is radiated as fluorescence and this fluorescence is detected to form an image. Specifically, there is known a radiation image conversion method employing stimulable phosphors, as described in, for example, U.S. Pat. No. 3,859,527 and Japanese Patent O.P.I. Publication No. 55-12144. In this method is used a radiation image conversion panel having a phosphor layer containing a stimulable phosphor. Radiation having passed through an object is irradiated onto the stimulable phosphor layer, in which radiation energy corresponding to radiation transmission densities of the individual portions of the object is accumulated, thereafter, the stimulable phosphor is excited in time series by an electromagnetic wave (exciting light) such as a visible ray or an infrared ray, whereby the radiation energy accumulated in the stimulable phosphor is emitted in the form of a stimulated luminescence. Signals of the thus emitted luminescence are, for example, photoelectrically converted to obtain electric signals. The thus obtained electric signals are reproduced as a visible image on a recording material such as a photosensitive material or on a display such as a CRT.
The radiation image reproduction as above possesses the advantage that a radiation image with extensive information is obtained through radiation exposure dose far lower than that of conventional radiography of the combination of a conventional radiographic film and an intensifying screen.
Such a radiation image conversion panel using a stimulable phosphor accumulates radiation image data and emits accumulated energy by scanning with an excitation light, so that accumulation of radiation images is again feasible after scanning, enabling repeated use. Thus, conventional radiography consumes radiographic films for every photographing but on the contrary, the radiation image conversion method, which repeatedly uses a radiation image conversion panel, is advantageous in terms of resource protection and economical efficiency.
Further, recent diagnostic image analysis requires a radiation image conversion panel of further enhanced sharpness. There were attempted means for improving sharpness, for example, controlling the shape of the formed stimulable phosphor particles to achieve improvement in sensitivity and sharpness. Such attempts included, for example, a method of using a stimulable phosphor layer formed of fine pseudo-columnar blocks deposited on a substrate having a fine convexoconcave pattern as described in Japanese Patent O.P.I. Publication No. 61-142497.
There were also proposed a method employing a radiation image conversion panel having a stimulable phosphor layer in which cracks between columnar blocks obtained by depositing a stimulable phosphor on a substrate having a micro-pattern were further developed by being subjected to a shock treatment, as described in Japanese Patent O.P.I. Publication No. 61-142500; a method employing a radiation image conversion panel in which cracking is caused on the surface of a stimulable phosphor layer formed on a substrate to form pseudo-columns (Patent Document 1); and a method employing a stimulable phosphor layer having voids which is formed on the substrate surface through vapor deposition and then subjected to a heat treatment to grow the voids to form cracks (Patent Document 2). There was also proposed a radiation image conversion panel having, on a substrate, a stimulable phosphor layer formed of long and thin columnar crystals exhibiting a given inclination to the vertical line of the substrate (Patent Document 3).
Recently, there was proposed a radiation image conversion panel using a stimulable phosphor comprised of an alkali halide such as CsBr as a main component, and activated by Eu. Specifically, the use of Eu as an activator enabled to render it feasible to achieve enhanced X-ray conversion efficiency which was never realized before.
There is known a radiation image conversion panel providing high luminance and reflecting excitation light or stimulated luminescence, employing, as a substrate, a highly-reflective material such as aluminum.
However, generally, a substrate for manufacturing a radiation image conversion panel has a large area (for example, 100×100 mm or larger), and is thin as compared with the size. When the substrate is supported with a support in a vapor deposition apparatus for deposition of a stimulable phosphor, the substrate sags downwards in the center under its own weight, and therefore, there is problem in that it is difficult to form a uniform stimulable phosphor layer on the substrate.
A radiation image conversion panel providing a radiographic image with high image quality is required in which a stimulable phosphor layer is formed through a combination of a reflection layer formation method, which provides a reflection layer between the substrate and a stimulable phosphor layer to efficiently emit stimulated luminescence from a stimulable phosphor layer, and a columnar structure formation method, which obtains a radiographic image with high sharpness by vapor-depositing a stimulable phosphor in the form of column on a substrate with many fine protrusions, i.e., a method of laminating a reflection layer on a substrate with a convexoconcave surface and providing on the substrate a stimulable phosphor layer comprised of a stimulable phosphor in the form of column.
In view of the above, there is proposed a radiation image conversion panel in which a reflection layer reflecting stimulated luminescence is formed on a convexoconcave surface including wall surface of the convexo portions of a substrate, a stimulable phosphor layer containing a phosphor in the form of column is formed on the reflection layer according to a vapor phase deposition method, and a protective layer is formed on the stimulable phosphor layer, the reflection layer is a laminated layer comprising a metal layer and a transparent thin layer (Patent Document 4).
However, the radiation image conversion panel as proposed above improves image quality to some level, but the improvement is still insufficient. There are problems in a single metal layer that many defects are likely to occur, and defects such as density irregularity unevenness in an image are likely to be produced.    Patent Document 1: Japanese Patent O.P.I. Publication No. 62-39737    Patent Document 2: Japanese Patent O.P.I. Publication No. 62-110200    Patent Document 3: Japanese Patent O.P.I. Publication No. 2-58000    Patent Document 4: Japanese Patent O.P.I. Publication No. 2004-163383